1. Field of the Invention
The invention relates to an image forming apparatus.
2. Description of the Related Art
Hitherto, in an image forming apparatus such as electrophotographic apparatus, electrostatic recording apparatus, or the like, after a toner image was transferred onto a sheet, the sheet is conveyed to a fixing device, and the toner image is fixed by heating and pressing it in the fixing device, thereby forming an image onto the sheet.
FIG. 13 shows a construction of such a conventional image forming apparatus. Reference numeral 100 denotes an image forming apparatus; 101 an image forming apparatus main body (hereinbelow, referred to as an apparatus main body); 102 an image forming unit; and 5 a fixing roller pair serving as a fixing apparatus.
The image forming unit 102 has: photosensitive drums (a to d) for forming toner images of four colors of yellow, magenta, cyan, and black; an exposing device 6 for forming an electrostatic latent image onto each of the photosensitive drums by irradiating a laser beam on the basis of image information; and the like. The photosensitive drums (a to d) are driven by motors (not shown). A primary charging unit, a developing unit, and a transfer charging unit (which are not shown) are arranged around each drum and have been constructed in unit shapes as process cartridges 1a to 1d. 
Reference numeral 2 denotes an intermediate transfer belt which is rotated in the direction shown by an arrow. By applying transfer biases to the intermediate transfer belt 2 by transfer charging units 2a to 2d, the toner images of the respective colors on the photosensitive drums are sequentially multiplex-transferred onto the intermediate transfer belt 2. Thus, a full-color image is formed on the intermediate transfer belt.
Reference numeral 3 denotes a secondary transfer unit for transferring the full-color image which have sequentially been formed on the intermediate transfer belt 2 onto a sheet P; 5 the fixing unit for fixing the image on the sheet onto the sheet P; and 11 a discharge roller pair for ejecting the sheet P on which the image has been fixed to a discharge tray 7.
The image forming operation of the image forming apparatus 100 constructed as mentioned above will now be described.
When the image forming operation is started, first, the exposing device 6 irradiates the laser beam on the basis of the image information which is supplied from a personal computer (not shown) or the like, and sequentially exposes the surfaces of the photosensitive drums (a to d) which have uniformly been charged to a predetermined polarity and a predetermined electric potential, thereby forming the electrostatic latent images onto the photosensitive drums. After that, the electrostatic latent images are developed by the toner and visualized.
For example, first, the laser beam according to an image signal of a yellow component color of an original is irradiated to the photosensitive drum (a) through a polygon mirror and the like of the exposing apparatus 6, thereby forming the yellow electrostatic latent image onto the photosensitive drum (a). The yellow electrostatic latent image is developed by the yellow toner supplied from the developing unit and visualized as a yellow toner image.
Subsequently, in association with the rotation of the photosensitive drum (a), when the toner image flows into a primary transfer unit with which the photosensitive drum (a) and the intermediate transfer belt 2 are come into contact, the yellow toner image on the photosensitive drum (a) is transferred to the intermediate transfer belt 2 by a primary transfer bias applied to the transfer charging unit 2a (primary transfer).
Subsequently, when a portion holding the yellow toner image on the intermediate transfer belt 2 is moved, a magenta toner image formed on the photosensitive drum (b) so far by a method similar to that mentioned above is transferred on to the yellow toner image on the intermediate transfer belt 2. Similarly, as the intermediate transfer belt 2 moves, a cyan toner image and a black toner image are overlaid and transferred onto the yellow toner image and the magenta toner image in the primary transfer unit, respectively. Thus, the full-color toner image is formed on the intermediate transfer belt 2.
In parallel with the toner image forming operation, the sheets P enclosed in a paper feed cassette 4 are fed out one by one by a pickup roller 8 and flow into a resist roller 9. After timing is matched by the resist roller 9, the sheet is conveyed to the secondary transfer unit 3. In the secondary transfer unit 3, the toner images of four colors on the intermediate transfer belt 2 are transferred onto the sheet P in a lump by a secondary transfer bias which is applied to a secondary transfer roller 3a (secondary transfer).
Subsequently, the sheet P to which the toner images have been transferred as mentioned above is guided by a conveying guide 20 provided between the secondary transfer unit 3 and the fixing roller pair 5 and conveyed to the fixing roller pair 5 constructed by a heating roller 5a and a pressing roller 5b. By the fixing roller pair 5, the sheet P is heated and pressed and fixed. Thus, the toners of the respective colors are melted and color-mixed and a full-color print image fixed to the sheet P is obtained. After that, the sheet P is ejected to the discharge tray 7 by the discharge conveying roller pair 11 provided on the downstream of the fixing roller pair 5.
In recent years, in the image forming apparatus, miniaturization and high speed of the apparatus are strongly demanded. In such an image forming apparatus, a technical problem which occurs frequently is a problem that in the fixing roller pair 5, the heat is applied to the sheet and the conveyed sheet becomes a heat source, thereby raising a temperature of the whole apparatus.
As another problem, there is a problem of an inter-sheet adhesion in which if the heat-applied sheets themselves are continuously delivered and stacked, the obverse surface of one sheet and the reverse surface of another sheet which faces the sheet are adhered. Such an inter-sheet adhesion is liable to occur in the case where heating performance of the fixing roller pair 5 (fixing device) is improved in order to improve fixing performance of the image on an OHT sheet, thick paper, or the like or the case where thin sheets of paper to which the duplex printing has been performed are continuously stacked.
Among the problems, a technique regarding how to effectively cool the sheet after the fixing becomes an important subject. To accomplish such a subject, hitherto, a cooling fan is arranged on a conveying path after the fixing, thereby cooling the heat applied to the sheet.
Further, for example, as shown in FIG. 13, a cooling roller pair 10 is arranged on the downstream side in the conveying direction of the fixing roller pair 5, the air is blown to the cooling roller pair 10 by a cooling fan (not shown), and the cooling roller pair 10 is cooled, thereby realizing a cooling effect of the sheet (JP-A-2004-109732).
Hitherto, as a fixing device for fixing the toner image onto the sheet, there is a heat roller fixing system in which the sheet is heated while being sandwiched with a pressure and conveyed by the heating roller (fixing rotary member) 5a held to a desired temperature and the pressing roller (pressing rotary member) 5b which is come into pressure contact with the heating roller 5a. There is also a fixing device in which a fixing belt, a heating film, or the like which is come into pressure contact with the pressing roller and rotated and has been heated by a heating source is used in place of the heating roller.
However, in such a fixing device, in the case where the sheets of a small size in which a length in the direction (hereinbelow, referred to as a width direction) which crosses perpendicularly the sheet conveying direction is shorter than that of the sheet of the maximum size are continuously fixed in a fixing region, a temperature of a non-paper-passage surface of the heating roller 5a excessively rises. This is because if the sheets of the small size are continuously allowed to pass, in the non-paper-passage region of the heating roller 5a where no sheet passes, the heat is partially accumulated because there is no heat-take-away by the sheet.
Such a phenomenon is called an edge portion temperature elevation or a non-paper-passage portion temperature elevation of the fixing device. If the temperature of the edge portion of the fixing device rises as mentioned above, a hot offset of the image to the heating roller occurs and, if the temperature exceeds a temperature elevation limit of the component elements of the fixing member, it results in a damage of the parts.
To prevent such a problem, therefore, in the conventional fixing device, self heat radiation cooling is executed for a predetermined time or until a value of a detection signal of detecting means for detecting the temperature of the heating roller or the pressing roller in the non-paper-passage region is equal to a predetermined value. After temperature distribution of the whole region in the width direction became almost uniform by such self heat radiation cooling, the next sheet is allowed to pass.
However, in order to make the temperature distribution of the whole region in the width direction almost uniform by performing the self heat radiation cooling, a cooling time of tens of seconds to about a few minutes, that is, a down-time is necessary and the next paper passage cannot be performed for such a down-time, so that the improvement of the productivity is obstructed.
Therefore, to prevent such a non-paper-passage portion temperature elevation, there has been known a construction in which a blowing fan is provided for the fixing device and the air is blown to the heating roller or the pressing roller in the non-paper-passage region, thereby suppressing the temperature elevation. Further, there is also a construction in which when the cooling air is blown to the non-paper-passage region side from the cooling fan, a length in the width direction of a ventilation port is adjusted in accordance with a width of sheet which is used, thereby preventing the non-paper-passage portion temperature elevation also to the sheets of different sizes (refer to JP-A-2003-076209).
However, in such a conventional image forming apparatus, for example, in the case where the air is blown by a cooling fan 170 and the cooling roller pair 10 is cooled as shown in FIG. 14, there is the following problem.
If the apparatus is miniaturized, particularly, in the case where the sheet P has been conveyed, the air after the sheet was cooled by the cooling fan 170 flows into the fixing roller pair 5 in dependence on the direction of a duct 171. If the air after the cooling flowed into the fixing roller pair 5, the temperature of the fixing roller pair 5 decreases and a heat generation amount of the heating roller 5a increases.
Further, as shown in FIG. 15, when the sheet is passing through the fixing roller pair 5, the air after the cooling collides with the sheet P and, thereafter, flows into the conveying path before the fixing through the outer periphery of the heating roller 5a. 
When the air flows into the fixing roller pair 5 as mentioned above, it is heated. Therefore, there is also a problem that, when the heated air flows into the image forming unit 102 as well as the secondary transfer unit 3 after that, the temperature of the image forming unit 102 rises and the toner is melted in the image forming unit 102.
Also in the case of taking a measure for the temperature elevation by cooling the non-paper-passage portion of the heating roller 5a of the fixing roller pair 5 by a cooling fan 172 as shown in FIG. 16, the cooling effect to the non-paper-passage portion cannot be sufficiently obtained in dependence on a way of blowing the cooling air and the productivity is deteriorated.
There is also a problem that the air after the non-paper-passage portion of the fixing roller pair 5 was cooled by the cooling fan 172 also flows into the conveying path on the upstream side more than the fixing roller pair 5 in dependence on the direction of a duct 173, an influence is exercised on the temperature elevation of the image forming unit, and the toner is melted in the image forming unit.